Bone Formation and Resorption
In the adult individual (males as well as females) bone is continuously subject to remodeling. This is a process where bone resorption is closely linked to bone formation, through the concerted action of the bone active cells, i.e. the bone forming osteoblasts and the bone resorbing osteoclasts. These cells together form what is called a basal multicellular (metabolic) unit, or BMU. The remodeling process starts with activation of the lining cells (the cells that cover the unmineralized bone). The lining cells resorb the unmineralized bone, then retract and leave room for the osteoclasts which resorb the old, mineralized bone and create an environment which attracts the osteoblast to the same site. The osteoblasts thereafter lay down the organic matrix, which subsequently is becoming mineralized to form new bone. The resulting bone mass is thus determined by the balance between resorption by osteoclasts and formation by osteoblasts.
Consequently, there is a close relationship between the actions of the two cell types which is referred to as “coupling”; bone resorption always precedes bone formation. The coupling phenomenon means that even when the intention is to produce a positive balance per cycle it is still necessary to start with bone resorption. Typically, a BMU cycle takes 3 to 6 months to complete.
The rate by which the basal metabolic (multicellular) units are being activated, the activation frequency, also plays a role. A high activation frequency increases the rate by which bone is being lost if there is a negative balance per remodeling cycle. When activation frequency is increased the space that is being occupied by remodeling, the remodeling space, is also increased. This will give a lowered bone mass, since a greater portion of the bone is subject to resorption as part of the remodeling process.
The above outlined sequence of events is well known in the art and has formed the basis for the understanding of metabolic bone diseases and possible ways for their treatments.
Osteoporosis is a disease which is characterized by a reduced amount of bone tissue, usually of normal composition, which has reduced strength due to a combination of low bone mass and impaired architecture, and therefore carries an increased risk of fractures. In terms of remodeling, osteoporosis is the result of negative bone balance per remodeling cycle, i.e. less bone is formed than is being resorbed. In a small proportion of patients it is possible to determine a specific disease as responsible for the loss of bone (e.g. malabsorption of calcium and hypersecretion of corticosteroid hormones) but in the majority of patients no such disorder is identified. Such patients are classified as having “primary” osteoporosis. Bone is lost with advancing age in both sexes, but in females there is generally an increased rate of loss during the first years after the menopause (hence the term “postmenopausal” osteoporosis).
Bone Resorption Inhibitors
A number of agents have been used for the prevention and treatment of bone loss and osteoporosis, e.g. estrogen, vitamin D and bisphosphonates, such as alendronate (for a review, see: Osteoporosis (Marcus, R., Feldman, D. and Kelsey, F., Eds.) Academic Press, San Diego, 1996). Such agents mainly act through inhibition of bone resorption. By reducing the resorbed amount in each remodeling cycle, while keeping the formation intact, it is possible to reduce the negative bone balance and retard bone loss. At the same time they reduce the activation frequency and since the remodeling space is reduced there is only a limited increase of bone mass.
Most studies with bisphosphonates indicate that they increase bone mineral density of the lumbar spine in the actively treated patients with around 1 to 5%, depending on dose and type of bisphosphonate, during the first year of treatment, when compared with control patients given placebo. Both patients and controls generally receive calcium supplementation to ensure adequate calcium nutrition.
The antiresorptive agents can retard bone loss but, by definition, they do not increase bone mass within each remodeling unit. Many patients with fractures have severe bone loss at the time they come to clinical attention. Inhibition of bone resorption might not be enough to prevent fracture recurrences. Therefore it is urgent to develop therapies that can increase bone mass, i.e. anabolic agents.
Parathyroid Hormone
Parathyroid hormone (PTH) is an 84 amino acid polypeptide which is normally secreted from the parathyroid glands. PTH has an important physiological role to maintain serum calcium within a narrow range. Furthermore, it has anabolic properties when given intermittently. This has been well documented in a number of animal and open clinical studies, recently reviewed by Dempster, D. W. et al. (Endocrine Reviews 1993, vol. 14, 690–709). PTH has a multitude of effects on bone. Part of it is through the remodeling cycle. PTH causes both increased activation frequency and a positive balance per cycle.
Human PTH may be obtained through peptide synthesis or from genetically engineered yeast, bacterial or mammalian cell hosts. Synthetic human PTH is commercially available from Bachem Inc., Bubendorf, Switzerland. Production of recombinant human parathyroid hormone is disclosed in e.g. EP-B-0383751.
PTH when given alone, to a patient with osteoporosis, will stimulate bone formation within each remodeling cycle and cause a positive bone balance within each cycle. At the same time the number of remodeling units will greatly increase, i.e. the activation frequency is enhanced. These two mechanisms act in different directions on bone mass.
During therapy with PTH it has been calculated that the activation frequency is doubled. Although this will mean that the remodeling space is increased, bone mass (or bone density) is increased in trabecular bone. Thus bone mineral density is increased by 5 to 10% per year in the lumbar spine and is largely unaffected in the femoral neck, which contains a higher proportion of cortical bone. These two sites are where the most common and clinically important fractures occur in the population, both in males and females.
The presently known methods for treatment of osteoporosis utilize bone resorption inhibition of the BMU cycle, but have the drawbacks that their onset of effect is slow and limited, and that they only cause moderate increases of bone mineral density (bone mass) and may therefore be insufficient for the treatment of patients with osteoporosis in a stage where there is high risk of recurrent fractures. Furthermore, it has not been shown that present methods can improve on the altered architecture that is a hallmark of advanced osteoporosis.
A method of treatment of bone metabolism disorders, utilizing the order of events in the BMU cycle, and comprising administering a bone active phosphonate and, sequentially, parathyroid hormone, is disclosed in WO 96/07417 (The Procter & Gamble Company). In that method, the bone active phosphonate is given for a period of greater than about 6 months, in various dosage regimens, but always prior to PTH.
Hodsman, A. et al. (J. Bone and Mineral Research, Vol. 10, Suppl. 1, abstract No. P288, p. S200, 1995) discloses a clinical trial involving treatment with PTH for 28 days, with our without sequential calcitonin for 42 days, with this cycle repeated at 3 months intervals for 2 years. Patients were then crossed over to clodronate, 28 days per 3 months, for one year. However, there was no beneficial effect in bone density from this sequential PTH/bisphosphonate treatment regimen.
WO 97/31640 (publication date 4 Sep. 1997) discloses a pharmaceutical composition comprising (a) an estrogen agonist/antagonist; and (b) a bone activating compound, such as parathyroid hormone. However, the periods of treatment are broadly defined and it is stated that the said compounds can be administered for periods from about three months to about three years.